1. Field of the Invention
The present invention relates to charge control agents, toners containing the charge control agents, and toner producing methods.
2. Description of the Related Art
The processes to develop electrostatic images using toners in image forming apparatuses like electrophotography are broadly classified into two; that is, processes that utilize two-component developers in which toners and carriers are mixed and processes that utilize one-component developers containing only toners. In the processes by use of the two-component developers, a toner and a carrier are stirred and charged into different polarities, and the charged toner visualizes electrostatic images having reverse polarity thereof; specific examples are magnet brush processes by use of iron-powder carriers, cascade processes by use of beads carriers and fur brush processes. The processes by use of the one-component developers are exemplified by powder cloud processes in which toners are used in a spray condition, contact development processes in which toners are directly contacted with latent electrostatic images and developed, non-contact development processes in which latent electrostatic images are developed by slightly separated toners, etc.
The toners applied in theses developing processes contain a colorant such as carbon black and pigments dispersed into binder resins. Magnetic toners are also conventional that contain these ingredients and further a magnetic material like magnetites. As described above, toners used in various developing processes are positively or negatively charged depending on the polarity of electrostatic images to be developed. In order to charge toners, the charging ability of resins in the toners may be made use of; however, the charging ability is typically insufficient when arising solely therefrom, thus the resulting images tend to be foggy and unclear. Therefore, charge control agents are typically included into the binder resins in addition to the colorants in order to provide toners with an intended charging ability. Conventional charge control agents with negative polarity are exemplified by metal complexes of mono azo dyes, nitrofumic acid and salts thereof; metal complexes of acids such as salicylic acid, naphthoic acid and dicarboxylic acid and metals such as zinc, aluminum, cobalt, chromium and iron; sulfonated copper phthalocyanine pigments, etc.
However, the charge control agents described above are typically of complex structure and inconsistent properties, lack stability, and often change their charging ability depending on environments. Moreover, most of the metal complexes are color compounds, which thus tend to affect adversely color reproducibility of colorants in developed images when used in color toners. Furthermore, compounds with heavy metals should often be addressed in terms of their influence on safety and environment.
Therefore, various polymer compounds have been proposed as charge control agents containing no metal. For example, Japanese Patent Application Laid-Open (JP-A) No. 63-184762 discloses a copolymer of styrene and acrylamide with a sulfonic acid group; JP-A No. 2003-98752 discloses a copolymer of a monomer with a sulfonic acid group and a halogen-substituted phenylmaleimide. However, the toners containing these polymer compounds are not sufficiently improved with respect to the tendency to decrease their charge amount under high temperature and high humidity conditions. In addition, JP-A No. 08-95305 reports that a copolymer of phenylmaleimide substituted by an electron attractive group such as halogen and nitro group is used for a charge control agent. However, this charge control agent is also insufficient in rating property of charging ability and stability of charge amount. Japanese Patent No. 3550638 discloses a copolymer of N-substituted maleimide and methylstyrene as a maleimide-styrene copolymer. The copolymer is disclosed merely as to a secondary nonlinear optical property with proper thermal stability.